Temperature compensated variable inductance



J. F. BELL Mrch 9, 1948.

TEMPERATURE COMPENSATED VARIABLE INDUCTANCE 2 sham-sheet 1 Filed Feb. 13, 1943 ,Z22/mx KZZ J. F. BELL 2,437,345

TEMPERATURE COMPENSATED'VARIABLE INDUCTANCE Filed Feb. 15, 1945 2 sheets-sneer 2 y... j s

7 mM w March 9, 1948.

Patented Mar. 9, 1948 TEMPERATURE COMPENSATED VARIABLE INDUCTANCE John F. Bell, Chicago, lll., assignor to Zenith Radio Corporation, Chicago, Ill., a corporation of Illinois Application February 13, 1943, Serial No. 475,779

9 Claims. (Cl. 171-242) This invention relates to a low frequency drift variable tuned circuit.

One of the objects of the invention is to provide an improved low frequency drift variable tuned circuit and particularly an oscillator circuit which maintains a practically constant lfrequency notwithstanding considerable variations of temperature.

A further object of the invention is to provide an improved variable inductor whi-ch has a very low change of inductance with change of temperature.

A further object of the invention is to provide an improved variable inductor in which the ratio of change of inductance with change of temperature is substantially proportional to the inductance for which the inductor may be set.

.A further object of the invention is to provide an improved tuned circuit including a variable inductor in which the ratio of change of inductance with change of temperature is small and is substantially proportional to the inductance for which the inductor may be set, together with capacitor means having a temperature coefcient, that is, a ratio of change of capacity with change of temperature, which substantially corrects for changes of inductance and capacitance with temperature to provide a tuned circuit having substantially negligible frequency drift with changes of temperature.

A further object of the invention is to provide an improved tuned circuit which can :be tuned to any desired frequency within an extended range of frequencies and which will maintain any desired frequency to which it may be tuned within close limits notwithstanding considerable change of temperature.

Other objects, advantages and capabilities of the invention will appear from the following description of a preferred embodiment thereof,

taken in conjunction with the accompanying drawings, in'which:

Figure 1 is a longitudinal sectional view of my improved Variable inductor;

Fig. 2 is a side elevation thereof;

Fig. 3 is an end elevation thereof;

Fig. 4 is a perspective view of one of the clips to rwhich the inductor coil is connected;

Fig. 5 is a plan view of the coil and a. form in which it may be mounted Fig, 6 is a similarview showing another coil form;

2 eating means for initial drift due to heating up of the tube;

Fig. 9 is a frequency-dial marking curve of the oscillator shown in Fig. 7 or Fig. 8; and

Fig. 10 is a frequency drift-frequency curve showing the negligible drift of my improved circuit.

My invention includes an improved variable inductor which is arranged to have a relatively low drift of inductance in all its settings and in which the drift of inductance with temperature change is substantially proportional to the inductance for which the inductor is set within its operating range. I iind that if such an inductor is incorporated as the variable element of a variable tuned circuit, the frequency drift has a straight line relation to temperature change and can be corrected by capacitor means having a suitable temperature coeiilcient.

My invention includes a tuned circuit embodying such an inductor and such capacitor means. My improved variable inductor, and consequently the tuned circuit, are arranged for accurate reproduction of tuning so that the same frequency may be attained accurately when the tuning means of the inductor is located Iin the same dial position.

My improved inductor is illustrated in detail in Figs. 1 to 6. The materials of which it is made are selected so that no changes of parameter result when the temperature of the inductor is raised or lowered and then returned to initial temperature. The metal parts of the inductor fulfill this requirement to a high degree. The coil forms IU and II are of dielectric material selected to meet this requirement. I nd that a coil form I0 of steatite has satisfactory expansion properties. Likewise, the coil form II of glass meets these requirements. Other dielectric substances have the same properties but the two mentioned have proved to be perfectly satisfactory.

My improved inductor comprises a main frame or base I2 which is preferably of metal, cast iron being a suitable material. The coil form I 0 is mounted between two upstanding lugs I3 and I4 which extend upwardly from the base I2, The lug I3 is provided with an opening I5 which receives a metal bushing I6 having a conedv face against which one end of the form I0 abuts as shown in Fig. 1.

The bushing I6 is rigidly held in desired adjusted position by means of a :binding screw I1 which extends through a slot I 8 in the upper end of the lug I3 as shown in Fig. 3. The other end of the form I0 abuts against the coned endof 3 a sleeve 'I 9 which is slidably mounted in an opening in the lug I4. The sleeve I9 abuts against a washer 20 carried by a gasket 2I of rubber or other insulating material. Thegasket 2| is held compressed by means of an internally threaded cap 22 which is mounted on a threaded nipple 23 iortending from the lug I4 away from the form A slug shaft 24 is slidably mounted in the bushing I8 and in a bearing 25 carried by an annulus 26 of compressed rubber or other dielectric located in an enlargement 21 .in the sleeve I9 remote from the form I0.

The slug shaft 24 is biased towards the left as viewed in Fig. 1 by means of a coil spring 28 which abuts against a nut 29 threaded on the end of the shaft 24 and against a washer 30 of insulating material which abuts against the face of the cap 22. It is to be noted that the shaft24 is electrically connected to the base I2 through the bushing I6, but the shaft is insulated from the lug I4 to avoid an electrical loop constituted by the shaft 24 and the frame I2 which would rey sult in excessive losses and uncertain operation.

The shaft 24 is arranged to be moved axially by means of a knob 3I which is mounted on a shaft 32 which carries a worm 33. The shaft 32 is rotatably mounted in a bearing 34 formed on a web 35 which extends upwardly from the frame I2. 'I'he web 35 is provided with a bore 36 which extends into the opening of the bearing 34. The bore 36'contains a. pressure pin 31 adapted to be forced against the shaft 32 by means of a set screw 38 threaded into the web 35, the pin 31 and the set screw 38 having cooperating coned ends as shown in Fig. l. By means of the set screw 38, a desired degree of friction may be applied to the shaft 32 so that it maintains the desired position in which the shaft 32 is moved by the knob 3|.

The inner end of the shaft 32 is supported by means of a ball 39 received in a recess in the end of the shaft 32 and in a. recess carried by a plug 48 which is threaded into a boss 4I carried by the frame l2.

The plug 48 is locked in position by means of lock nut 42. The shaft-32 has a shoulder which abuts against the inner side of the bearing 34.

The worm 33 meshes with a Worm gear 43 rigidly carried by a sleeve 44 which also rigidly carries a pinion 45 at the level of the shaft 24. The sleeve 44 is rotatably mounted on a post 46 which projects upwardly from the base I2. The pinion 45 meshes with a rack 41 formed on the righthand end of shaft 24 as viewed in Fig. l. Consequently, rotation of the knob 3I eects axial movement of the shaft 24 in one direction or the other.

At the right-hand end of the inductor as viewed in Fig. 1,` the base I2 is provided with an upstanding lug 48 which has an opening through which the right-hand end of the shaft 24 extends. A key plate 49 adjustably mounted on the lug 48 bears against the shaft 24 to limit lateral motion thereof. At the same time, engagement of the key end of the plate with a keyway (not shown) in the end of the shaft limits rotary motion of the shaft. Beyond the lug 48 the shaft 24 is reduced and threaded to receive a scale 50 of rectangular form and a securing nut 5I.

The longitudinal member of the scale 50 is provided with suitable divisions and numerals which are arranged to cooperate with an index mark 52 on the web 35. Also mounted on the lug 48 is an angular resilient clip 53 which cooperates with the set screw 38 to prevent accidental loss or loosening thereof.

The coil form I Il shown in Figs. l, 2 and 5, is suitably of steatite or similar material, and it may be of generally dumbbell form. The coil 54, Which is relatively short, is tightly wound in a helical groove formed in the central part of the form I0, its ends being soldered to clips 55 which are mounted on the enlarged ends of the form I0 by means of screws 56. A

To avoid excessive drift dueI to expansion and contraction of the coil 54 with temperature changes, I prefer to make this coiI of Invar wire which is copper coated and preferably also silver coated to provide a low resistance layer. The coil 54 is tightly wound upon the form I0 at a low temperature so that it is always in firm contact with its groovev in the form. The expansion of the coil 54 is consequently the expansion of the material of which the form I0 is made.

As shown in Fig. 6, I may employ a glass form II upon which are mounted clips 51 for connection of the ends of the coil. Between these clips the coil is provided with a helical groove 58 for the reception of the tightly wound coil 54.

The shaft 24 carries an iron slug 58 and a conductive slug 60. To attain my objects of keeping inductance drift with temperature low, and keeping the drift proportional to the inductance, I employ these two slugs so as to get a large travel of the slug shaft. I use the term slug to designate generally a. body movable relative to the coil to effect a change of its inductance.

I use powdered -iron of such characteristics that, upon temperature change, the inductance always returns to its initial value lupon the return of the temperature to its initial value. When such iron is utilized, the permeability is suiiiciently high to make the slug of satisfactorily small diameter. The slug'59 may'suitably consist of this powdered iron cemented to slug form fby a suitable binder such as a thermal setting plastic. The slug 59 is suitably cemented to the shaft 24.

The slug 60 consists primarily of copper. To minimize the drift at one position of adjustment of the shaft 24, I provide at the outer end of this slug an annulus 6I of metal having a smaller thermal expansion than that of copper. Various metals having different thermal expansions may be used to attain any desired correction of drift which may be necessary. In the present embodiment of the invention the annulus 6I may suitably be of Invar or of otherv similar metal having a very small thermal expansion. The composite slug is secured to the shaft 24 by means of a -pin 62. Annulus'GI is'plated with copper and preferably also with silver, so that it has high frequency electrical properties of copper orl silver.

While I have referred to the element 60 as a slug, it must be understood that only the oute1` surface layer is effective electrically, land at high frequencies this layer is extremely thin. Thus, the Invar annulus 6I has no electrical function. It merely provides a support for the copper or silver surface and imparts to the copper or silver cylinder a much lower thermal expansion than that which the cylinder would have if unsup ported. The slug 68, 6I may be hollow or it may The shait 24 is suitably of stainless steel. 4The portion of the shaft between the two slugs 59 and 60 may be platedor coated with a non-magnetic metal of high conductivity such as copper or silver, or both. Instead of so coating this portion of the shaft, I may provide a sleeve extension 53 of the slug 60 which extends up to the slug 59. When the shaft 24 is thus coated, it has substantially no effect upon the electrical qualities of the coil 54.

The slugs 59 and 69 are in suchspaced relation that as one leaves the coil 54 electrically, the other enters the coil electrically. Actually .the slugs are spaced slightly less than the physical length of the coil. It is to be noted that the slug 60 is shown with a slight conical formation on its inner end, but the ends of :the slugs may be varied in shape to some extent without affecting the operation of the inductor provided the spacing of the slugs is made correct as hereinafter explained.

The electrical properties of my improved inductor can best be understood with reference to a tuned circuit in which it is incorporated. In Fig. 7 I show a wiring diagram of a Colpitts oscillator including the coil 54 tuned by the slugs 59 and 60 to determine the frequency of oscillation.

'Ihe B battery is connected through a resistor 64 and through a chokev coil 65 to the grid side of the coil 54, the other side thereof being connected to the plate of the tube 6G by a conductor 6l.

A voltage regulator tube 68 which may suitably be a VR-150 tube, is connected to the common point of the resistance 64 and choke coil 85 and to ground so as to establish substantially constant voltage on the plate of the tube 66.

The grid-to-ground condenser 69 may suitably be several times larger than the plate-to-ground condenser 10. Last said condenser may have a value of around 100 ya farads. The condenser 'I0 may suitably be a ceramic condenser having a suitable temperature coeiiicient, usually a negative temperature coefficient, suflicient to correct for frequency drift due to changes in capacity in the circuit with temperature and due also to changes of inductancein the inductor including the coil 54. This inductor has only a slight change of inductance with temperature change and that change of inductance with temperature change is substantially proportional to the inductance for which the inductor is set. Consequently, this slight change of inductance can be compensated f-or, together with drift due to capacity, by a condenser 'I0 of correct temperature coeiiicient.

The correct temperature coefficient to be selected for the condenser l0, can be determined empirically in the manner set forth in my U. S. Patent No. 2,371,790, dated March 20, 1945. In short, the method consists in installing a condenser 10 of known temperature coe'icient and determining the frequency, with one setting of the slug shaft at two considerably different ambient temperatures.

From the frequency change it can be determined that an additional change of capacity of determined amount in capacitor 10 would bring the drift to zero. To this change yof capacity is algebraically added the actual change introduced by the condenser 'Ill and from this result it is a matter of elementary calculation to determine that the condenser 'l0 should, for correction of ambient temperature, have a definite temperature coemclent. Correction for ambient temperature drift is thus attained by substituting a condenser 10 of the same capacity and having the desired temperature coefficient. A

In Fig. 9 I have shown a graph of oscillator frequency against dial markings derived from such a circuit. The part of the curve correspond-- ing to between 0 and 12 on the dial marking corresponds to the movement of the iron slug from the position shown in Fig. 1, that is, when it is fully inside the coil l54, to a position in which it just emerges from this coil. Change of frequency has a straight-line relation to dial marking for the greater parts of its length, but adjacent the 161/2 mc. point, the curve tends to atten out as shown by the dotted line 1|.

The slope of the curve depends primarily upon the diameter of the slug. If the slug 59 were made of larger diameter, the straight-line part of the curve between 131/2 mc. to ll/2 mc. would be steeper. The same applies to .the conductive slug B0 with respect to the portion of the curve between about 17 mc. and 20 mc. The diameters of the two slugs are thus correlated so that they provide two portions of the curve of .the same slope.

Correct alignment of the two curves is attained by accurate spacing of the two slugs. Thus, if .the slug 60 were moved slightly to the right, portion of the curve of Fig. 9 between 17 mc. and 20 mc. would be moved downwardly on this figure.

Correct spacing'places the two portions of the curve in alignment. It is to be noted that below around 17 mc. point, the upper part of the curve, due to the slug 60 alone,` tends to flatten out as shown by the dotted line l2. The two dotted lines 'H and 12 combine to form a complete straight line.

It is to be understood that it is within the scope of my invention to construct an inductance in which the inductance varies non-linearly in a desired fashion upon adjustment of the two relatively movable parts thereof. That is, as set forth above, the two slugs 59 and 60 may be made of s'uch relative diameters, various sections of each of the two slugs 59 and 60 may be made of different relative diameters, and the spacing between the two slugs 59 and 60 may be such as to obtain a very large variety of non-linear curves of induction upon change in adjustment of the two relatively movable parts.

In Fig. 10 I have shown thefrequency drift of a circuit as described above, expressed in cycles, against the frequency expressed in megacycles. From this curve, it is obvious that the frequency drift lies within the limits of i0.01% of the frequency for a variation of i475 C., these limits being indicated by the dotted lines 'i3 on Fig. 10.

It is to be noted that when .the slug 59 is within the coil 54, the frequency diminishes 20 cycles per temperature rise of one degree centigrade. As the iron slug is withdrawn from the coil, the frequency drift increases slightly and then it decreases until at a point where the slug 59 is almost out of the coil, the frequency drift is 0.

As the slug 60 is drawn into the coil, the fre- 7 l end by the Invar element 6| and I nd that with this Invar element, the frequency drift between 19 and 21 mc. is practically zero.'

It must be emphasized that the full line curve shown in Fig. 10 can, for all practical purposes, be regarded as a straight line. The drift there indicated, amounting' to less than 30 cycles in about 15,000,000 cycles for each degree centigrade, is negligible for all practical purposes. The dial may be set to a desired frequency with the aid of the Vernier scale on the knob 3l with an accuracy of about one kilocycle. At the worst part of the curve shown in IFig. 10, one kilocycle corresponds to drift due to 30 C., so that the maximum frequency drift is of substantially the same order as the accuracy of setting the dial and its eil'cct is completely negligible. The dial can be set to a desired position and a station of the corresponding wave length can be received under all possible conditions of temperature.

It has been explained that the condenser l having an appropriate temperature coefficient, substantially compensates for drift due to changes of capacity and inductance due to change of ambient temperature. If desired, the tuned circuit may be also corrected for frequency changes due to warming up of the tube 66. This is done in the manner described in detail in my aforesaid copending application, and it will therefore be referred to only briefiy herein with reference to Fig. 8.

For this result, I connect in parallel with the heater of the tube 66, a heater 18 having a'relatively long and stout lead or conductor 'I1 which is grounded to the chassis. Instead of the single condenser "I0, I` employ a condenser .10 in the same relation and a condenser 18 in parallel thereto, these two condensers having the same total capacity as the original single condenser. The temperature coeilcients of the two condensers is such' that they supply the same capacity change with change of temperature as did the original single'condenser. I

The grounding of the condenser 'I8 is effected by a relatively ne conductor 'I9 which is connected tothe conductor 11 at the point 80, The point 80 and the effective length of the conductor 'I9 are determined in the manner set forth in the aforesaid copending application so that the condenser 18 heats up as the tube 66 heats up and at a rate which compensates for change lof frequency due to change of parameters of the tube 66 during heating. With this compensating system, the frequency remains substantially constant right from the instant that the tube 68 heats up enough to be effective.

It is to be noted that on increase of temperature, the cast iron frame I2 between the post 46 and the lug I3 expands, thus tending to move the coil form I0 to the left relative to the slugs, thus increasing the frequency. The much lesser expansion of the material of the coil form I0 tends to have the same effect. The expansion of the stainless steel shaft 24 tends to move the slugs 59 and 60 to the left, as viewed in Fig. 1. 'Ihe stainless steel shaft is selected of material which has a thermal expansion intermediate those of the base I2 and the form I0, and such that the expansion of the base I2 and the form I0 are offset to some extent by the expansion of the shaft 24.

The expansion of the portion ci the shaft 24 between its point of engagement with the pinion 45 and the effective slug, that is, the slug which is cooperating with the coil 54, is somewhat less than the sum of the expansion o! the frame between the pin 46 and the abutment I3 and the expension of the half of the coil form I0 between this abutment and the coil. Consequently, an increase of temperature has the effect of pulling the iron slug 59 out of the coil 54 to a slight extent, thus tending to increase the frequency. The expansion of the shaft 24 is selected so that this slight net movement of the slug 59 relative to the coil 54 substantially balances the ,elect on the inductance of temperature changes of the iron slug 59 and the coil 54.

Since these materials have excellent expansibility and contractibility with change of temperature, the rate of change of inductance with temperature change resulting from these factors is highly ,constant at any setting of the inductance.

I may, by proper selection of thermal expansions and dimensions of the coil form, frame and slug shaft, make. the drift of inductance with change of temperature, when the slug 59 is cooperating with the coil, substantially zero. In practice, it is not necessary to compensate to this extent because I find that if the inductance drift with temperature change is made small, it is substantially proportional to the inductance setting, and it can be balanced by a temperature compensating condenser, Since I prefer to use such a condenser to compensate for drift due to changes of capacity of the circuit with change of temperature, it is convenient to employ a condenser of appropriate temperature coeiiicient to correct for both the residual drift of the inductance and the drift of capacity due to change of temperature.

Expansion of the iron slug with increase of temperature tends to decrease the frequency in the case of most iron slug compositions. This effect, however, is minimized since the slug 59 is relatively small. My employment of an iron slug and a slug of conductive metal enables me to have a relatively large travel of the slug shaft, which materially reduces the drift ofinductance in frequency due to temperature change.

Expansion Aof the coil 54 with increase of temperature tends to decrease the frequency. This effect is kept at a minimum by employing, in effeet, a coil which has a very low expansibility. In fact, it has the expansibility of the form which is quite low, compared with that of -most normal metals. The Invar coil 54 is wound upon the form at a very low temperature and under substantial tension. This tension is retained at all times with the result that the Wire of the coil is rmly seated in its groove at all times.

The tendency toward increase of frequency resulting from the relative displacement between the coil 54 and the slug 59 owing to the expansion of the coil form I0, the frame I2 and the shaft 24, is arranged so that it substantially nullies the tendency towards decrease of frequency resulting from the simultaneous expansion of the coil 54 and the slug .59. While this result can be conveniently attained by selecting a. shaft 24 of correct thermal expansion, it is obvious that the effect involves the thermal expansion of the coil form I0 and the frame I2 and the same result can be obtained by selecting material for the coil form I0 or material for the frame l2 so that the three elements I0, I2 and 24 will provide the small displacement of the slug 59 relative to the coil 54 necessary effectively tonullify the frequency change resulting from expansion of the slug 58 and the coil 54.

Expansion of the slug 60 tends to increase the frequency with rise of temperature. During the first part of the travel of the slug this tendency minimizes a tendency toward opposite drift, but a slug 60 completely of copper gives excessive increase of frequency with rise of temperature when the slug is more than half way into the coil. As shown by the portion of the dotted line 14 above the upper dotted line 13 in Fig. 10, the frequency drift exceeds 0.01% of the frequency for a change of temperature of @17.5 C. when more than about three-quarters of the slug 60 is within the coil. `By substituting the portion 6l of low thermal expansion for part of the slug 60, it will be seen that the frequency drift when more than about one-third of the slug 601s within the coil,

is reduced to not more than 0.0019 of the frequency for a change of temperature of $47.5" C., a drift which is substantially negligible.

It will be understood that although I have referred to Invar as a prime example of metal having substantially no thermal expansion, it is well known in the arts that many other metals and alloys have the same or similar property, and they may be used instead of Invar in the relations herein stated. It will be understood that in some cases a lesser degree of correction than that provided by the Invar element 6| is necessary and that the element 6| may in such cases be made of a metall which has a substantial expansion somewhat less than that of copper. It will also be understood that the length of the element 6| and its position on the slug 60, 6l is determined by the position in the tuning range where the correction it provides is necessary.

It must also be understood that it is within the scope of the present invention to provide a slug such as 60, 6I, to correct for a reduction of frequency with rise of temperature, and in such cases an element 6 I may be provided at the proper position on the slug 60, 6l, which has a greater thermal expansion than that possessed by the body 60 of the slug.

While it is preferred in the embodiment of the invention illustrated to employ two slugs 59 and 60, it will of course be apparent that each slug exercises its individual tuning effect and lt is within the scope of my invention to employ a single iron slug 59 or a single -composite slug 60, 6I to provide an inductor having a lesser tuning range. p

Although the invention has been described in connection with specific details of preferred embodiments thereof, it must be understood that such details are not intended to be limitative of the invention except in so far as set forth in the accompanying claims.

I claim:

1. A variable inductance comprising a coil, a conductive non-magnetic slug arranged for movement into and Within said coil, one part of said slug being of metal having a definite thermal expansion and another part being of metal of different thermal expansion, said slug being adapted to influence the inductance of the 'coil progressively as it is moved into the coil.

2. A variable inductance comprising `a coil, a slug arranged for movement into and within said coil, one end of said slug being of conductive metal expansible with rise of temperature and the other end being of metal substantially nonexpansible with rise of temperature, rcoated with a metal of high conductivity, said slug being adapted to influence the inductance of the coil progressively as it is moved into the coil.

3. A variable inductance comprising a coil, a. dielectric form on which said coil is wound, a frame carrying said form having an abutment against which bears one end of said form the cooperating surfaces of said abutment and 'said one end of said form being surfaces of revolusaid form against said abutment to move saidv form into a position with its axis in alignment with the axis of said surfaces on said frame, a shaft carried by said frame disposed along the axis of said surfaces on said frame and passing axially through said form, and slug means carried by said shaft for varying the inductance of the coil according to the position of said slug'means relative to said coil.

4. A variable inductance comprising a coil having a very small temperature coefficient of expansion, a frame, an abutment on the frame, means supporting the coil on the frame with said abutment iixed with respect to said coil and the rest of the frame free to move axially of the coil upon thermal expansion and contraction, a movable shaft passing axially through said coil and supported on said frame with a point on said frame on the contra coil side of the abutment determining the position of the frame relative to said shaft under temperature changes, slug means carried by said shaft and positioned along the axis of said coil according to the position of said shaft for influencing the inductance of said coil dependent upon the relative axial position of said means and said coil, said frame and said shaft each having a temperature coefficient of expansion much larger than said coil, said shaft having a lesser coefficient than said frame and of such magnitude as to substantially reduce any motion of said slug means relative to said coil upon change in temperature.

5. A variable inductance comprising a dielectric form having a very small temperature coefiicient yof expansion and having a helical channel therein, a coil of wire of even smaller temperature coeicient of expansion lying in tension in'said channel, a frame, an abutment on the frame, means supporting said form on said frame and resiliently pressing the form axially against said abutment a' shaft passing axially through said coil and movably mounted on said frame,

means for moving said shaft axially through said coil, said last mentioned means including means on the contra coil side of said abutment determining the position of said shaft relative to said frame, and slug means carried by said shaft for varying the inductance of said coil dependent upon the position of said shaft relative to said coil, said frame and said shaft each having a temperature coefficient of expansion much larger than said form, said shaft havin'g a lesser coeillcient than said frame and of such magnitude as to substantially reduce any motion of said slug means relative to said coil' and the residual motion of said slug-means relative to said coil being in such direction as to substantially eliminate changes in inductance of said coll due to second order of magnitude changes in the size of the coil and said slug means upon change of temperature.

6. A variable inductance including a coil, a magnetic slug'a-nd a conductive slug, said slugs beingconnected for movement in unison along the axis of said coil, said coil with the magnetic slug lying therein having a iirst ratio of inductance change to inductance for a predetermined temperature change, said conductive slug being formed vof material having characteristics which vary with temperature in such amount that the ratio of inductance change to inductance for a predetermined.temperature change with said conductive slug lying in said coil is substantially the same as said first ratio.

7. A variable inductance including a framework, said framework including a coil form, a coil carried by said coil form, a shaft adjustably supported from said framework and positioned substantially coaxially in said coil, a conductive tuning slug and a magnetic tuning slug supported on said shaft and movable thereby into and out of said coil, the coil with said magnetic slug lying thereinhaving a rst ratio of inductance change to inductance for a predetermined temperature change, said conductive slug being formed `of material having characteristics which vary with` temperature in such amount that the ratio of inductance change to inductance for Va predetermined temperature change with said conductive slug lying in'said coil is substantially the same as said iirst ratio.

8. A variable inductance including a framework, said framework including a coil form, a coil carried by said .coil form, a shaft adjustably supported from said framework and positioned substantially coaxially in said coil, a conductive slug and a magnetic slug supported on said shaft and movable thereby into and out of said coil,

said coil with said magnetic slug lying therein having a first ratio of inductance change to inductance for a predetermined temperaturev change, said conductive slug being formed of a plurality of materials having composite characteristics which Vary with temperature in such l amount that the ratio of inductance change toV inductance for, la predetermined temperature change' with said conductive slug lying in said coil is substantially the same as said rst ratio.

9. A variable inductance including a framework, said framework including a coil form, a

12 coil carried by said coil form, a shaft adjustably 'supported from said framework Vancl positioned substantially coaxially in said coil, a conductive slug and a magnetic slug supported on said shaft and movable thereby into and out of said coil, said coil with said magnetic slug lying therein having a rst ratio of inductance change to inductance for a predetermined temperature change, said conductive slug being formed of a plurality of materials having composite characteristics which vary with temperature in such amount thatl the ratio of inductance change to inductance ,for a predetermined temperature change with said conductive slug lying in vsaid coil is substantially the. same as said iirst ratio, the material of said conductive slug being Invar and copper. i

JOHN F. BELL.

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